Sodium hydrogencarbonate (NaHCO3 which is also called baking soda or sodium bicarbonate) is widely used in the field of various food products, as ingredient of baking powder, an additive for soft drinks, etc., in the pharmaceutical industry as a dialysate, an antacid, etc., and further as a fire-extinguishing agent, as a bath additive, as a detergent, as a blasting medium, as an acidic gas neutralizing agent, etc. In most cases, such sodium hydrogencarbonate is produced and packaged, delivered, transported, stored and used in the form of powdery or granular crystal particles.
However, commercial crystal particles of sodium hydrogencarbonate generally show a caking property and has a high caking property resulting from sodium carbonate formed by drying in the production process, especially in an environment at a high temperature with a high humidity. This tendency is remarkable particularly in the rainy season. If caking occurs, the flowability of the particles will be low, and the handling efficiency will deteriorate remarkably in the respective steps from distribution to use, and various troubles are brought about. Thus, the caking is a serious problem which may impair the commercial value of sodium hydrogencarbonate.
Heretofore, in order to prevent the caking of sodium hydrogencarbonate crystal particles, e.g. Patent Document 1 proposes to incorporate various anticaking agents, such as stearates, carbonates, phosphates, silicates, kaolin, talc or silicon dioxide.
However, such a conventional method for incorporating an anticaking agent, not only requires a cost for the anticaking agent or a step of its incorporation, but also requires selection of the type of the anticaking agent depending upon the particular application. Further, for food products, pharmaceuticals, etc., it is impossible to use it, or its amount of use may be limited.
Further, Patent Document 2 discloses a method for producing sodium hydrogencarbonate crystals having a reduced caking property, by drying sodium hydrogencarbonate at from 20 to 60° C. by heated air. However, since the drying temperature is low, the treatment efficiency of a drying apparatus tends to be low, the drying apparatus tends to be large, or the drying time tends to be long.
Further, Patent Document 3 discloses a method for producing sodium hydrogencarbonate crystals having a reduced caking property, which comprises making sodium sesquicarbonate be present on the surface of sodium hydrogencarbonate crystal particles. However, in order to convert sodium hydrogencarbonate to sodium sesquicarbonate, treatment with a high humidity in a long time is required, and accordingly the equipment tends to be large. Further, in the case of treatment at a temperature of at least 30° C. to shorten the treatment time, the temperature and the humidity at the time of conversion into sodium sesquicarbonate greatly vary depending upon the carbon dioxide gas concentration in the atmosphere, and accordingly the carbon dioxide gas concentration, the humidity and the temperature must be controlled with very high accuracy.
Patent Document 1: JP-A-5-58622
Patent Document 2: Japanese Patent No. 3306873
Patent Document 3: JP-A-2003-104722